In process automation technology as well as in manufacturing automation technology, field devices are often applied, which serve for registering, and/or influencing, process variables. Serving for registering process variables are measuring devices, or sensors, such as, for example, fill level measuring devices, flow measuring devices, pressure, and temperature, measuring devices, pH, and redox potential, measuring devices, conductivity measuring devices, etc., which register the corresponding process variables, fill level, flow, pressure, temperature, pH-value, or conductivity. Serving for influencing process variables are actuators, such as, for example, valves or pumps, via which the flow a liquid in a pipeline section, or the fill level in a container, can be changed.
Classified as field devices are, in principle, all devices, which are applied near to the process and which deliver, or process, process-relevant information. Besides the aforementioned measuring devices/sensors and actuators, also classified as field devices are, generally, also units, which are connected directly to a fieldbus and serve for communication with superordinated units. Thus, also classified as field devices are e.g. remote I/Os, gateways, linking devices and wireless adapters. A large number of such field devices are produced and sold by the Endress+Hauser Group.
In modern industrial plants, field devices are, as a rule, connected with superordinated units via fieldbus systems, such as e.g. Profibus®, Foundation Fieldbus®, HART®, etc. Normally, the superordinated units include control systems, or control units, such as, for example, a PLC (programmable logic controller). The superordinated units serve, among other things, for process control, process visualizing, and process monitoring, as well as for start-up of the field devices. The measured values registered by the field devices, especially by sensors, are transmitted via the connected bus system to one, or, in given cases, also to a number of superordinated unit(s). Along with that, also data transmission from the superordinated unit via the bus system to the field devices is required; this serves, especially, for the configuring and parametering of field devices or for diagnostic purposes. In general, the field device is serviced via the bus system from the superordinated unit.
Besides hardwired data transmission between the field devices and the superordinated unit, there is also the opportunity for radio, or wireless, data transmission. Especially in the bus systems, Profibus®, Foundation Fieldbus® and HART®, wireless data transmission via radio is provided for. Additionally, radio, or wireless, networks for sensors are specified in greater detail in the standard IEEE 802.15.4.
For implementing wireless data transmission, newer field devices, especially sensors and actuators, are embodied, in part, as radio field devices. These include, as a rule, a radio unit and an electrical current source as integral components. In such case, the radio unit and the electrical current source can be provided even in the field device or in a radio module connected durably to the field device. Through the electrical current source, a self-sufficient energy supply of the field device is enabled.
Along with that, there is the opportunity to transform field devices without radio units—thus the installed base—into a radio field device by coupling such with, in each case, a wireless adapter, which has a radio unit. A corresponding wireless adapter is described, for example, in the publication WO 2005/103851 A1. The wireless adapter is, as a rule, releasably connected to a fieldbus communication interface of the field device. Via the fieldbus communication interface, the field device can send the data to be transmitted via the bus system to the wireless adapter, which then transmits the data via radio to the desired location. Conversely, the wireless adapter can receive data via radio and forward such via the fieldbus communication interface to the field device. The supplying of the field device with electrical power occurs then, as a rule, via an energy supply unit of the wireless adapter.
In the case of autarkic radio field devices with or without wireless adapter, communication, for example, with a superordinated unit is conducted, as a rule, via the wireless communication interface of the radio field device, or via the wireless adapter, as the case may be. Additionally, such radio field devices, or wireless adapters, have, as a rule, a hardwired communication interface. For example, it is provided in the HART® standard that radio field devices must have, besides a wireless interface, also a hardwired communication interface. Via such a hardwired communication interface, for example, on-site configuration of the radio field device, or of the wireless adapter, is possible via a service unit, such as, for example, a handheld communicator, which is connected to the hardwired communication interface. Additionally, the hardwired communication interface can be embodied as a fieldbus communication interface, so that communication thereover is conducted corresponding to a bus system, such as, for example, corresponding to one of the standardized bus systems, Profibus®, Foundation Fieldbus® or HART®. Via such a fieldbus communication interface, the radio field device, or the wireless adapter, can also be connected to a corresponding hardwired fieldbus. The energy supply unit, or the electrical current source, of wireless adapters, or of a radio field device, is usually a onetime use battery or a rechargeable battery.
In the case of field devices, for which only a limited energy reserve is available, it must be heeded, that no energy is consumed unnecessarily. An effective method for energy saving in the case of two wire, or radio, field devices is to provide the field devices with the ability to move between two operating phases, an active phase and a resting phase. During the resting phase, the energy supply to the individual system components is reduced, or the components are turned off. Often, one speaks, in this connection, of switching the field devices into a sleep mode.
The large part of the installed base of field devices today are HART devices, thus field devices, which communicate via the HART standard with a superordinated control unit. These field devices must be able, at any time, to react to communication requests of the control unit. Such communication requests occur, for example, in the case of a configuring or parametering of the field device or in the case of the sending off of a control command to the field device, or the control unit asks for a measured value query from the field device, in order to obtain information concerning the present measured value. Of course, the configuring of the field device, comparatively speaking, does not occur very often. Depending on application, the measured value query likewise occurs relatively greatly spaced in time. If the field device would, despite the sporadic communication requests, keep the communication relevant components continually supplied with energy and ready, then, a large part of the time, energy would be wasted. In the case of e.g. battery operated field devices, or field devices, for which only a limited amount of energy is available via a two-wire line, such a waste of energy is not acceptable. In the case of a supplying of the field device from a onetime use battery, battery life declines. In the case of a two wire supply, the energy used for the continual communication readiness cannot be used by the other components of the field device.